Reverse input blocking clutches are configured to transmit rotation of an input-side member to an output-side member when input torque is applied to the clutch, and prevent rotation of the input-side member if reverse input torque is applied to the clutch. One type of such reverse input blocking clutches are configured to lock the output-side member if reverse input torque is applied (this type of reverse input blocking clutches are hereinafter referred to as “locking type (reverse input blocking) clutches”), and are frequently used when it is required that the output-side member maintain its position even after the motor that applied input torque to the input-side member stops, or if the motor stops when it is not supposed to, due e.g., to power outage.
Among such locking type reverse input blocking clutches, one type of well known clutch includes a torque transmission arrangement disposed between an input-side member and an output-side member which are configured to rotate about a common axis, and the torque transmission arrangement is configured to transmit rotation of the input-side member to the output-side member with a slight angular delay. A fixed outer ring has a cylindrical surface on the inner peripheral side thereof and disposed radially outwardly of the output-side member. The output-side member has an outer peripheral surface formed with a plurality of cam surfaces such that a wedge-shaped space which gradually narrows toward its respective circumferential ends is defined between the cylindrical surface of the fixed outer ring and each of the cam surfaces of the output-side member. A pair of rollers as engaging elements and a spring as an elastic member are mounted in each wedge-shaped space such that the rollers are pushed into the respective narrow end portions of the wedge-shaped space. A retainer is provided which includes pillars inserted in both circumferential end portions of the respective wedge-shaped spaces, and which is coupled to the input-side member so as to rotate in unison with the input-side member (see, for example, Japanese Patent 4965871B).
With this type of reverse input blocking clutch, since the rollers are pushed into the narrow end portions of the respective wedge-shaped spaces under the biasing force of the springs, if reverse input torque is applied to the output-side member, the rotationally rearward rollers engage the fixed outer ring and the output-side member, thereby locking the output-side member, so that the input-side member never rotates.
On the other hand, when input torque is applied to the input-side member, the pillars of the retainer, which rotate in unison with the input-side member, push the rotationally rearward rollers into wide portions of the wedge-shaped spaces against the biasing force of the springs. This disengages the rollers from the fixed outer ring and the output-side member, thus unlocking the output-side member, so that rotation is transmitted from the input-side member to the output-side member through the torque transmitting means. At that time, since the rotationally forward rollers are moved, relative to the output-side member, to the wider portions of the respective wedge-shaped spaces, the rotationally forward rollers will never engage the fixed outer ring and the output-side member.
Typically, in such locking type reverse input blocking clutches, in order to prevent separation of the rollers in the axial direction, the retainer is arranged to restrain the axial movements of the rollers toward one axial end of the clutch, and the fixed outer ring has a lid portion integral with the fixed outer ring and covering the other ends of the wedge-shaped spaces to prevent the axial movements of the rollers toward the other axial end of the clutch.
In this arrangement, while rotation is being transmitted from the input side to the output side, since the rollers revolve around the axis of the clutch together with the input-side member and the output-side member, the rollers slide on the inner peripheral cylindrical surface of the fixed outer ring and the opposed surface of the lid portion opposed to the rollers. Thus, the sliding resistance between the rollers and the fixed outer ring and between the rollers and the lid portion adds to the torque necessary to transmit rotation. To reduce power consumption, specifically the power consumption of the motor which applies input torque to the input-side member, it is necessary to reduce the torque necessary to transmit rotation.
One way to reduce the torque necessary to transmit torque would be to reduce the forces of the springs biasing the rollers, thereby reducing the sliding resistance between the rollers and the fixed outer ring and between the rollers and the lid portion. However, since the forces of the springs affect the locking performance, it is impossible to indefinitely reduce the forces of the springs. That is, if the forces of the springs are too small, it becomes more difficult, if reverse input torque is applied, to bring the rollers into engagement with the fixed outer ring and the output-side member, which could result in the output-side member being not locked.